1. Field of the Invention
The present invention is directed to a manner of achieving rigidity and dimensional stability in a heat dam strip section for use in an electric fuse of the time delay type.
2. Description of the Prior Art
It is well known in the prior art to make use of a heat dam section in an electric fuse to reduce the flow of heat from the fusible element to the end terminals of the fuse. Heat dams typically are formed from sheet metal and comprise a series of U-shaped bends interconnecting a plurality of planar sections to define a tortuous path. Such structure results in a relatively heavy fuse link structure, or one involving a relatively large mass, to achieve a given current rating. Consequently, the lag-times or time delay of such fuses are particularly long making them extremely attractive for application in motor circuits having high motor starting inrush currents.
Such heat dam elements are shown and described in U.S. Pat. No. 3,261,950 issued July 19, 1966, entitled TIME-LAG FUSES HAVING HIGH THERMAL EFFICIENCY, U.S. Pat. No. 3,261,952 issued July 19, 1966, entitled TIME-LAG FUSE WITH RIBBON FUSE LINK HAVING TWO SYSTEMS OF BENDS, and in U.S. Pat. No. 4,216,457 issued Aug. 5, 1980, entitled ELECTRIC FUSE HAVING FOLDED FUSIBLE ELEMENT AND HEAT DAMS.
Typically in such an electric fuse the heat dam sections are made from materials such as copper or commercial bronze and, in some fuse applications, the thickness of the material from which the heat dams are fabricated may be as little as several thousandths of an inch. It has been found that heat dam sections which have been made from very thin sheet material are susceptible to being deformed from their desired shape during handling of the heat dams and various sub-assemblies which include the heat dams during the assembly of a fuse.
Deformation during handling could, for example, result in a pair of adjacent planar sections being extremely close to or actually touching one another. In this situation, it would be possible that an insufficient quantity of pulverulent arc-quenching and cooling filler material would find its way into the region between these planar sections during the filling operation. As a result, during use of the fuse, this region of the heat dam could become a hot-spot and cause undesirable and premature opening of the fuse in the heat dam.